Project summary/abstract Worldwide, millions of people suffer from respiratory lung diseases. The only viable option for patients with the end-stage pulmonary disease is lung transplantation, a mediation that is impeded by a major shortage of donor lungs. Current bioengineering strategies to generate a fully functional lung have been largely unsuccessful in reproducing its extraordinary complexity; achieved by a step-wise developmental program of the lungs in vivo. Recent studies highlight the contribution of cell competition in the developmental program of organogenesis, a still poorly understood mechanism that can be used to reconstitute a particular tissue or organ otherwise unable to form properly. From this perspective, Blastocyst Complementation (BC) is a highly innovative regenerative approach that allows integration of donor cells in defective host animals resulting in the generation of chimeric animals. This strategy has been developed to regenerate functional internal organs, such as the kidney and pancreas in small and large animals. Utilizing this approach, we successfully generated normal lungs by rescuing the lung agenesis phenotype of a Fgfr2 null mutant mouse in vivo. The recipient mice survived to adulthood with fully functional regenerated lungs. These observations give rise to crucial questions: how this cell competition mechanism acts in different defective progenitor niches, what is the impact of specific signaling pathways in the context of different defective organ niches, and how do we optimize these findings. To gain insights into these issues, we propose to: (Aim1) Examine BC-mediated mechanisms of cellular reconstitution in defective lung mesodermal and endodermal organ niches. (Aim2). Explore how donor cells are targeted into the lung organ niche in vivo. These studies will contribute to a better understanding of the mechanism of organogenesis and will establish a solid base for the future use of BC-based approaches in regenerative medicine.